This is probably just me being paranoid, but I worry about performing this query every (health check) interval as well as the schedule queries - when I was testing stuff for a the cass healthcheck, I think I found that I couldn't guarantee that LIMIT 1 does the minimal work possible. 

But the scheduler already fetches the list of events - would it make sense to cache the oldest one that it gets, and see if that passes the threshold?  Since the scheduler runs probably more frequently than the health check, the scheduler could just cache the health of the last run it made (and the time?)  That way there are less queries against Cassandra?